Automatic signal bias correction



June 4, 1946.

G. L. ERICKSON AUTOMATIC SIGNAL BIAS CORRECTION Filed May 16. 1944INVENTOR G.L.ERICKSON ATTORN EY Patented June 4, 1946 AUTOMATIC SIGNALBIAS CORRECTION George L. Erickson, Hasbrouck Heights, N. J., assignorto The Western Union Telegraph Company, New York, N.

York

Application May 16,

13 Claims.

The invention relates to a method and means for compensating for signalbias which may be introduced in telegraph signals due to variation inleakage resistance or reduced insulation values of the line occasionedby fog, wet weather or other adverse conditions, and more particularlyto compensate automatically for spacing bias eifects appearing insignals repeated by a relay in a half-duplex telegraph circuit.

Long line telegraph circuits which are satisfactory during favorableweather conditions are apt to become difiicult to operate during wetweather periods due to leakage of current from the line conductor toearth or to other conductors. Ordinarily, circuits over which signalsare sent by reversals of current are not so seriously affected by suchconditions as are circuits over which the signals are sent by making andbreaking the circuit. It often proves advantageous, therefore, in thecase of long Morse or telegraph printer lines, to so arrange thetelegraph apparatus that polar transmission is used over the longersections, although the operators or subscribers at the various stationsmay send and receive single current signals on Morse sets or telegraphprinters.

Generally, in such arrangements the operators equipment, While initiallygenerating single current signals, is also operating a polechanger relaywhich in turn is sending reversals of current to the line and to adistant polar relay capable of being actuated by reversals of current. Alocal circuit at the distant terminal reproduces the single currentsignals originally generated by the other station for actuating thereceiving Morse or teleprinter apparatus. To make the circuit operativein both directions, recourse is had to duplex equipment, and in the caseof telegraph printers it is often desirable to operate them by means ofhalf-duplex polar operation in which sending is in but one direction ata time. In such arrangements marking current is transmitted against thereceived signals, and if wet weather occurs and the ohmic balanceordinarily obtained by an artificial line network is not corrected, thenmarking current to the line increases, causing a spacing bias to appearin the received signals.

An object of the invention is an improved method and means forautomatically compensating for bias that appears in telegraph signalsdue to variation in leakage resistance of the telegraph line.

A further and more specific object is to provide means for automaticallycompensating for the spacing bias which normally appears in the signalsrepeated by a relay in a half-duplex circuit due to variation in leakageresistance of the telegraph line.

Another object is to maintain centered the signals on trunks or otheropen fire circuits with-i Y., a corporation of New 1944, Serial No.535,814

out the necessityof maintaining an accurat ohmic balance of theartificial line networks associated therewith.

Other objects and advantages will appear from 5 the following detaileddescription of an illustrativ embodiment of the invention, taken inconnection with the accompanying drawing which shows a half-duplexteleprinter circuit embodying the principles of the invention.

In the circuit illustrated there are shown two identical sending andreceiving stations A and B connected by a telegraph line L. Each of thestations comprises, among other elements, a main line receiving polarrelay MLR,, and a loop or subscribers lin ID from a telegraph printer T,the printer having suitable sending contacts II for energizing apolechanger relay PCR. Each station also has a control relay CR which isenergizable by the receiving relay MLR, and an artificial line networkAL for balancing the telegraph line L in known manner. The telegraphline is'connected to one of the operating windings l5 of the main linerelay MLR, another Winding l6 of the relay being connected through theartificial line network AL to ground. The artificial line network has aline rheostat l8 for obtaining a preliminary ohmic balance, and othersections comprising adjustable resistances l9 and adjustable condensers20 for obtaining capacity balance.

The apex of the windings l5 and 16 of relay MLR is connected preferablyto an anti-noise, low pass filter comprising an inductance 24 and acapacity 25, and thence by conductor 26 to an armature 28 of thepolechanger relay PCR. Armature 28 has a biasing spring 29 which tendsto hold the armature against the back contact of the relay whendeenergized, which back contact has positive or spacing polarity appliedthereto from any suitable source, as from a generator 30. When relay PCRis operated, the armature 28 is attracted to the front contact of therelay, which front contact has negative or marking polarity appliedthereto, as from a 45 generator 32.

One end of the winding of relay PCR is connected to the teleprinter dropor line In, the other end of the winding being connected by a conductor34 to an armature 35 of the control relay CR.

50 A biasing spring 36 tends to hold armature 35 away from the groundedcontact 31 of the relay when the relay is deenergized. One end of thewinding of relay CR is connected to ground; the

other end is connected bya conductor 40 to the marking contact m of therelay MLR. The armature 4| of the latter relay is connected to a sourceof current 42 which may be either positive or negative.

When transmission is not taking place in either direction, steadymarking potential, of negative polarity in a conventional system, isapplied to 3 both ends of the circuit L, and the v tongues 4| of thepolar relays MLR remain in their marking positions m. At such time thecon-.

trol relay CR at each station is operated by a circuit which may betraced from ground. at one. end of its winding, conductor 40, markingcontact control grid 63 is connected by a conductor 66 to the anode orplate 68 of a rectifier, which may m and armature 4| to a source ofenergizing current 42. Relay PCR, also is operated over a one,

cuit which comprises a source of energizing current l2, which may beeither positive or negative,

closed contacts of the teleprinter T, conductor l0, winding of relayPCR, conductor 34 and armature 35 and contact 31 of operated relay CR toground. The negativemarking current maintained on the line is suppliedfrom the source 32, through the armature 28 and front contact ofoperated relay PCR, conductor 26, inductance 24 of the filter circuit,and winding l of relay MLR. Assume now that the teleprinter T associatedwithone of the stations, such as station A, starts transmission, inwhich case a spacing signal is transmitted over line L from station A,This is effected by reason of the fact that the operating circuit forthe polechanger relay PCR isopened by the operation of the teleprinter,and armature 28 of the relay is actuated against the back contact of therelay so that spacing battery is supplied from the source 30 throughthe'armature 28, conductor 26, inductance 24 of the filter, and coil l5of relay MLR to the line L. Since the signaling current from station Apasses through the winding I6 of relay MLR in a direction to neutralizethe current passing through winding IS, the armature 4| of relay MLR atstation A remains in itsmarking position. At station B-the spacingsignal of positive'polarity'passes through the winding l5 of the mainline relay and thence through filter circuit 24, conductor 26, armature28 and front contact of relay PCR to the negative source of current 32to ground. The spacing signal causes the armature 4| of relay MLR atstation B to be actuated to its spacing contact s, and this interruptsthe energizing circuit for control relay CR. Armature 35 of relay CE isactuated, by spring 36, Outer engagement with the grounded contact 31,thereby opening the circuit comprising conductor 34 and conductor In tothe teleprinter T associated with station B, thustransmitting a spacingor start signal to the teleprinter.

A following marking signal received from the line L causes the tongue ofthe relay MLR at station B to return to its marking contact.Transmission from the teleprinter T associated with station E to theteleprinter of station A is effected in like manner.

Each of the relays MLR also has-an auxiliary compensating windingcomprising coils 44 and 45 which are in inductive relation with windingsl5 and I6. Preferably, and as shown, the compensating winding isconnected'by a conductor 48, WhlCh passes through the normally closedcontacts of a test jack J to enable readings of the compensating currenttobe taken, and thence by conductor 5| to a bias compensator C, the lineL also being connected by a conductor 52 to the bias compensator. Theother'end of the compensating winding is connected to ground at 49. Inthe embodiment illustrated, conductor 5| is connected to the'slider of'avariable rheostat 60 which is in circuit with the anode or plate 6| ofthe amplifier section of an electron discharge tube 53 which preferably,although not necessarily, comprises in the same envelope arectifierand'an amplifier element. The tubepreferably,

' be the rectifier section of the tube shown. Conductor 66 also extendsto a storage circuit comprising a condenser 69 which is shunted by ahigh resistance 10 for purposes hereinafter set forth.

The cathode 64 of the amplifier is connected to the slider of apotentiometer 12 for controlling the cathode potential. One end of thepotentiometer is connected to the heating filament 65 of the amplifier;the other end is connected through a current limiting resistance 13 to asource of negative battery supply. The cathode 15 of the rectifiersection of the tube is connected through a high resistance 16 and theconductor 52 to the line L. A heating filament I4 is provided toactivate the rectifier cathode 15.

The bia compensator illustrated automatically compensates for thespacing bias which normally appears in signals repeated by the main linerelay of a half-duplex circuit of the character illustrated when leakageor escape of current from any cause takes place from the line L toground, for example, as a result of fog, wet weather or similar adverseconditions. The compensation provided is not a correction of the ohmicbalance and therefore obviates the necessity for attempting to. readjustthe artificial line network AL as weather changes or other variation inleakage occurs. The difficulties of attempting to compensate, during wetweather, by adjusting the network will be apparent, since such anadjustment usually requires interruption of the circuit for aconsiderable period of time, and in the meantime it is possible that agreat many distorted signals will have been received. In accordance withthe instant invention, the auxiliary compensating windings 44 and 45 ofthe main line relay are connected in series and are caused to carry acurrent in a direction to pro duce a suitable marking bias in relay MLR.The value of this current is controlled so that in dry weather, with anaccurate balance of the artificial line obtaining, it is very low andhas practically no effect on the performance of the main line relay,since under thi condition no compensation is required. As the currentflowing to the line increases, however, because of wet weather or otherleakage conditions, the compensating current in the auxiliary windingsis increased to counteract the spacing bia produced by the increasedmarking current on the line, and the ohmic balance normally provided bythe artificial line is not altered when the bias compensator is in use.

The automatic control of the compensating current i obtained byutilizing the voltage existing between the incoming line L and groundwhen both ends of the circuit are applying negative marking battery tothe line. Under this condition the voltage of the line with respect toground is negative and ha a higher value than is the case when thedistant end is on pacing. When the home end spaces, the voltage of theline becomes positive with respect to ground. The voltage from line toground, when both ends of the circuit mark, varies with the presence ofleakage on the line, being of highest negative value when there is noleakage and becoming lower in value, although remaining negative, asleakage increases. In the bias compensator the change in this value ofvoltage between line and ground is used to change the voltage on thegrid of the vacuum tube 58. As the negative potential of the line withrespect to ground decreases as a result of the leak on the lineincreasing, the negative voltage on the grid 63 of the tube with respectto its cathode 64 decreases, and this permits the plate current of thevacuum tube to increase. This plate current is passed through theauxiliary winding 44 and 45 of the relay MLR and compensates for thespacing bias caused by the increase in line current.

When the half-duplex circuit is idle, steady marking current is appliedat both ends of the circuit, and the line to ground voltage could beused directly to control the compensating current. During periods ofsignaling from either end, however, it is necessary to provide meanswhereby the bias compensator can pick out the voltage existing under themarking against marking condition. This is done by two features of thearrangement illustrated. The first of these is the insertion of therectifier between the line and the compensator connected so that therectifier conducts only when the potential of the line is negative withrespect to ground. Thus, whenever the home station spaces, making theline L positive with respect to ground, the compensator is cut off fromthe line. Since, with the home end marking, the voltage between lineandground is greater when the distant end marks than when it spaces, apeak voltage detecting and storing arrangement may be used to pick outand store the higher voltage. This may comprise condenser 68, which isshunted by the high resistance in the specific circuit illustrated thecondenser may he of the order of 4: microfarads, and the resistance maybe of the order of 20 megohms. The condenser 69 is charged by thehighest negative volt-age appearing between line and ground anddischarges slowly through the high resistance hunt whenever thecompensator is cut off from the line by the rectifier section of thetube, which occurs Whenever the voltage between line and ground fallsbelow the voltage to which the condenser is charged. In ordinaryteleprinter signaling, the marking against marking condition occursfrequently enough to maintain the charge on the condenser approximatelyat the highest voltage.

The characteristics of the vacuum tube employed in the bias compensatorpreferably are such that, when used on an average line, the compensatingcurrent is of the correct value with no resistance in the plate circuitof the tube except the relay windings. Under some conditions, generallywhere a considerable amount of cable separates the set with which thecompensator is used from the open line on which the leakage occurs,additional resistance may be required in the plate circuit, and this maybe eifected by adjustment of the plate rheostat B0.

In order to place the cathode 64 of the tube at the proper potentialwith respect to the grid 63, which is at peak negative line potential,the cathode is connected to the slider of the potentiometer 72 which isinclined in the heater circuit of the tube. The heater circuit issupplied from a suitable source of negative current, such as a 160 voltline battery preferabIy through a suitable current limiting resistance13. The cathode is, therefore, at a negative potential with respect toground and may be adjusted by means of the potentiometer 12 to a propervalue with respect to the grid of-the tube to limit the compensatingcurrent to a very low value in dry weather. Since the cathode is at arather high negative potential with respect to ground, the plate circuitmay terminate at ground potential, as shown. The plate will then be at apositive potential with respect to the cathode, and underproperconditions of grid bias plate current will flow.

The compensator may be connected to and removed from'the'set with whichit is used either by the provision of separate disconnecting plugs orswitches, in a manner well-known in the art.

The initial adjustment of the compensator may be made when the line L isdry. A good balance is first taken on the set; with the resistance ofthe rheostat 60 cut out, that is, in its 0 position, an adjustment ofthe cathode potentiometer-.12 will produce a variation of thecompensating current, as may be seen from a local milliammeter whenconnected into the compensating circuit jack J. The cathodepotentiometer is then set so that the compensating current is justsufficient to be read on the meter, usually not over two and one-halfmillamperes. On most lines this will be the only adjustment required,the plate rheostat remaining at 0. The accuracy of compensation may bechecked by observing the received line current on marking and spacingsignals from the distant end of the circuit at a time when leakage ispresent on the line. The value of the compensating current ordinarilyshould be approximately one-half of the difference between the receivedmarking and spacing currents. on some lines, generally where aconsiderable amount of cable separates the ofiice where the compensatoris used from th leaky line, it may be found that when leakage occurs thecompensating current will be too high, and an adjustment of the platerheostat 60 will take care of this condition. Such an adjustment shouldbe made in very wet weather, and once made may be left untouched forless severe conditions. Should the compensator set be transferred toanother line, changes in the settings of the potentiometers 60 and 12may be required.

The specification so far ha described the application of v the unbalancecorrecting device to the relay at the receiving end ofthe circuit, wherethe device applies a marking bias in opposition to the spacing bias,which tends to arise as wet weather leakage causes an increase in"current from the terminal setinto the line. Consider, however, thecondition which prevails when'the half-duplex terminal set corrected asdescribed turns to sending. Under this condition the tongue of the relayMLR. should remain fixed on' its marking contact under all conditions ofthe armature ofthe sending polechanger. The unbalancecorrector,.however, continues to function and to supply a marking biawhich serves to hold the armature of relay MLR more securely on its marking contact and unaifected by any unbalance component.

When the line is afiected by leakage to ground because of wet weather,the peak voltage occurring between lineand ground decreases roughly onevolt for each mil of resultant unbalance. In the particular circuitillustrated, the constants of the tube circuit are adjusted so thatforeach mil of unbalance a mil of correcting current is caused to flowthrough the auxiliary winding 44 and 45. This adjustment will, ofcourse, be varied to suit individual circumstances, such as large cablesections at one end or the other, and once adjusted is corrected foratypical distributionof grounded telegraphy lines.

escape. The operating potentials of the bias corrector are so chosenthatthe device primarily is responsive to the peak potential between theline and ground of the marking signals. This 'adjust-' ment largelyexcludes the effects of the interference currents which are usuallypresent on For brevity in the claims, the term instant is employed torefer to the voltage or potential obtaining on the line with respect toground under the signaling conditions specified therein.

Various modifications of the circuit arrangement and apparatus shown,and various equivalents or substitutes of the devices illustrated, willreadily occur to those versed in the art without departing from thespirit and scope of the present invention. For example, and by way ofillustration only, separate tubes may be employed instead of thecomposite tubes 58, and .other known types of amplifier, relay andrectifier devices suitable for the purpose may be employed instead ofthose shown. Also, the compensator may be employed with variousdifferent kinds of telegraph lines, either of one wire or two wiretypes, and either grounded or metallic circuits. Moreover, thecompensator connections may comprise known types of detachable plug orswitch arrangements so that one compensator may be employed at varioustimes with different telegraph circuits. The disclosure, therefore, isfor the purpose of illustrating the principles of the invention which isnot to be regarded as limited except as indicated by the scope of theappended claims.

I claim: a

1. A telegraph system comprising a telegraph line subject to variableleakage resistance, sending apparatus for transmitting marking andspacing signals over said line, receiving apparatus connected to theline and responsive tothe marking and spacing signals, and means forautomatically compensating for bias which appears in the incomingsignals due to variation in leakage of the line, said means comprising awinding inductively coupled to said receiving apparatus, and meansresponsive to and controlled by the instant voltage existing between theline and ground for producing in said winding a compensating current ofa magnitude and in a direction to counteract the bias appearing in saidsignals due to varying leakage resistance, said means -maintaining thevalue of said compensating current continually proportional to thevoltage existing between the line and ground during the reception ofmarking signals by said receiving apparatus.

2. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacingsignals over said line, a repeating relay having a winding connected tothe line and responsive to the marking and spacing signals, and meansoperatively associated with the repeating relay for automaticallycompensating for bias which appears in thesignals due to variation inleakage of the line, said means comprising a biasing wind-' inginductively coupled to the winding of said repeating relay and meansresponsive to and controlled by the instant voltage existing between theline and ground for producing in said biasing winding a compensatingcurrent of a magnitude and in a direction to counteract the biasappearing in said signals due to varying leakage resistance.

3. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacing polarsignals over said line, receiving apparatus connected to the line andresponsive to the marking and spacing signals, and means operativelyassociated with the receiving apparatus for automatically compensatingfor spacing bias which appears in the signals due to variation inleakage of the line, said means comprising a biasing winding inductivelycoupled to said receiving apparatus and means responsive to andcontrolled by the instant voltage existing between the line and groundfor introducing in said winding a compensating current of a magnitudeand in a direction to produce a marking bias to counteract the spacingbias appearing in said signals due to varying leakage resistance.

4. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacing polarsignals over said line, receiving apparatus comprising a half duplexrepeater connected to the line and responsive to the marking and spacingsignals, and means operatively associated with said repeater forautomatically compensating for spacing bias which appears in the signalsdue to variation in leakage of the line, said means comprising a biasingwinding inductively coupled to said repeater and means responsive to andcontrolled by the instant voltage between the line and ground forintroducing in said winding a compensating current of a magnitude and ina direction to produce a marking bias to counteract the spacing biasappearing in said signals due to varying leakage resistance.

5. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacingsignals comprising negative and positive current pulses, respectively,over said line, receiving apparatus connected to the line and responsiveto the marking and spacing signals, and means operatively associatedwith said receiving apparatus for automatically compensating for spacingbias which appears in the signals due to variation in leakage of theline, said means comprising a biasing winding inductively coupled tosaid receiving apparatus and means responsive to and controlled by thevalue of the instant negative voltage on the line with respect to groundfor introducing in said winding a compensating current of a magnitudeand in a direction to produce a marking bias to counteract the spacingbias appearing in said signals due to varying leakage resistance.

6. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacing pulsesignals over said line, receiving apparatus connected to the line andresponsive to the marking and spacing ignals, and means operativelyassociated with the receiving apparatus for auto matically compensatingfor bias which appears in the incoming signals due to variation inleakage of the line, said means comprising a winding inductively coupledto said receiving apparatus and means responsive to and controlled bythe instant value of the negative voltage on the line with respect toground for producing in said winding a compensating current which variesin magnitude as the instant value of said negative voltage on the linevaries, which compensating current flows in a'direction to counteractthe bias appearing in said signals due to varying leakage resistance.

7'. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacing polarsignals over said line, receiving apparatus connected to the line andresponsive to the marking and spacing signals, and means operativelyassociated with the receiving apparatus for automatically compensatingfor bias which appears in the incoming signals due to variation inleakage of the line, said means comprising a winding inductively coupledto said receiving apparatus and means responsive to and controlled bythe instant value of the negative voltage on the line with respect toground for producing in said winding a compensating current whichincreases and decreases in magnitude as the instant value of saidnegative voltage on the line decreases and increases, which compensatingcurrent flows in a direction to counteract the bias appearing in saidsignals due to varying leakage resistance.

8. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacing polarsignals over said line, receiving apparatus connected to the line andresponsive to the marking and spacing signals, and means operativelyassociated with the receiving apparatus for automatically compensatingfor bias which appears in the incoming signals due to variation inleakage of the line, said means comprising an electron discharge tubehaving an output electrode connected to a winding inductively coupled tosaid receiving apparatus and a control electrode in said tube energizedin accordance with the instant value of the negative voltage on the linewith respect to ground, for producing in said winding a compensatingcurrent which varies in magnitude as the instant value of said negativevoltage on the line varies and which flows in a direction to counteractthe bias appearing in said signals due to varying leakage resistance.

9. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacing polarsignals over said line, receiving apparatus connected to the line andresponsive to the marking and spacing signals, and means operativelyassociated with the receiving apparatus for automatically compensatingfor bias which appears in the incoming signals due to variation inleakage of the line, said means comprising an electron tube amplifierhaving an output circuit connected to a winding inductively coupled tosaid receiving apparatus and a control grid in said tube energized inaccordance with the instant value of the negative voltage on the linewith respect to ground, for producing in said winding a compensatingcurrent which varies in magnitude as the instant value of said negativevoltage varies and which flows in a direction to counteract the biasappearing in said signals due to varying leakage resistance.

10. A telegraph system comprising a telegraph line subject to variableleakage resistance, apparatus for transmitting marking and spacing polarsignals over said line, receiving apparatus connected to the line andresponsive to the marking and spacing signals, and means operativelyassociated with the receiving apparatus for automatically compensatingfor bias which appears in the incoming signals due to variation inleakage of the line, said means comprising an electron discharge tubehaving an output electrode connected to a winding inductively coupled tosaid receiving apparatus and a control electrode in said tube energizedin accordance with the instant value of the negative voltage on the linewith respect to ground for producing in said winding a compensatingcurrent which varies in magnitude as the instant value of said negativevoltage on the line varies and which flows in a direction to counteractthe bias appearing in said signals due to varying leakage resistance,and means for causing said compensator to respond only to the voltageexisting under a marking against marking condition on said linecomprising a rectifier interposed between the line and compensator andconnected so that the rectifier conducts only when the potential of theline is negative with respect to ground.

11. A telegraph system comprising a telegraph line subject tovariable-leakage resistance, apparatus for transmitting marking andspacing polar signals over said line, receiving apparatus connected tothe line and responsive to the marking and spacing signals, and meansoperatively associated with the receiving apparatus for automaticallycompensating for bias which appears in the incoming signals due tovariation in leakage of the line, said means comprising an electrondischarge tube having an output electrode connected to a windinginductively coupled to said receiving apparatus and a control electrodein said tube energized in accordance with the instant value of thenegative voltage on the line with respect to ground for producing insaid winding a compensating current which Varies in magnitude as theinstant value of said negative voltage on the line varies and whichflows in a direction to counteract the bias appearing in said signalsdue to varying leakage resistance, means for causing said compensator torespond only to the voltage existing under a marking against markingcondition on said line comprising a rectifier interposed between theline and compensator and connected so that the rectifier conducts onlywhen the potential of the line is negative with respect to ground, and apeak voltage storage device comprising a condenser shunted by animpedance, said storage device being connected to the output circuit ofsaid rectifier and to the control electrode of said discharge tube.

12. The method of compensating for spacing bias introduced in telegraphmarking and spacing signals due tovariation in leakage of a telegraphline over which the signals are transmitted to signal repeatingapparatus, which comprises generating a compensating current ofa'magnitude dependent upon the instant negative voltage on the line withrespect to ground, and inductively applying said compensating current tothe repeating apparatus in a direction to counteract the bias appearingin the signals repeated by said apparatus.

13. The method of compensating for spacing bias introduced in telegraphmarking and spacing signals in a half duplex circuit due to variation inleakage of a telegraph line over which the signals are transmitted torepeating relay apparatus, which comprises applying marking current ofnegative polarity to said line, gencrating a compensating current of amagnitude dependent upon the instant value of the negative voltage onthe line with respect to ground, and inductively applying saidcompensating current to the repeating relay apparatus in a direction tocounteract the spacing bias appearing in the signals repeated by saidapparatus.

GEORGE L. ERICKSON.

